Liquid crystal display

ABSTRACT

A liquid crystal display including: a first transparent substrate coated with a first alignment layer, a second transparent substrate coated with a second alignment layer, the second substrate facing the first transparent substrate, a liquid crystal layer between the substrates, a polarizer attached on the outer surfaces of the substrates, a pair of electrodes formed on the first substrates, and a driving circuit applying signal voltage to the electrodes. The liquid crystal molecules adjacent to the first substrate is rotated by applying the voltage, but, the liquid crystal molecule adjacent to the second substrate is fixed regardless of the applied voltage. The electrode pair, substantially straight data and common electrodes, are inclined at an angle with respect to a gate line.

[0001] This application claims the benefit of Korean Application No.10152/1996 filed on Apr. 4, 1996, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a liquid crystal display, andmore particularly to an IPS (In-Plane Switching) liquid crystal displaythat is large in area and exhibits a wide viewing angle.

[0004] Conventional TFT LCD (thin film transistor liquid crystaldisplay) has a drawback known as a viewing angle dependency, that is,the contrast ratio is changed according to the viewing angle. This hasmade it difficult to apply the technology to a large size display.

[0005] To solve this problem, various liquid crystal displays areproposed such as a retardation attaching TNLCD (twisted nematic liquidcrystal display) and a multi-domain liquid crystal display. These LCDsstill have other technical problems such as complicated productionprocess and shifting color tones.

[0006] 2. Discussion of Related Art

[0007] Recently, IPS LCD has been introduced to obtain a wide viewingangle. This technology is discussed in JAPAN DISPLAY 92, p547, Japanesepatent application No. 7-36058, Japanese patent application No.7-225538, and ASIA DISPLAY 95, p707. As shown in FIG. 1a and FIG. 1 c,in the liquid crystal layer 12 the molecules are aligned at a 45° angle.The principle transmittance axis of a polarizer 9 attached to the firstsubstrate 1 is the same direction as the alignment direction of theliquid crystal 12, and the principle transmittance direction of ananalyzer 10 attached to the second substrate 5 is perpendicular to thealignment direction of the liquid crystal layer 12. A pair of electrodes2,3 is formed on the first substrate 1.

[0008] In FIG. 1b and FIG. 1d, when the voltage is applied between twoelectrodes, a horizontal electric field is created. Therefore, thetransmittance is controlled by causing the liquid crystal molecules tobe rotated to be parallel with the electric field. When the rotationangle of the liquid crystal molecules is 45° in the normally black mode,the retardation value (Δnd) is about λ/2(0.21−0.36 μm) for a maximumtransmittance.

[0009] In conventional IPS LCDs as described above, the transmittance iscontrolled by birefringence, and a retardation film is necessary tocompensate for the viewing angle which increases the manufacturing cost.

[0010] In addition, a viewing angle inverted area appears in the centralportion of the outer lines of the display.

SUMMARY OF THE INVENTION

[0011] Accordingly, the present invention is directed to a liquidcrystal display that substantially obviates one or more of the problemsdue to limitations and disadvantages of the related art.

[0012] An object of the present invention is an IPS mode liquid crystaldisplay having a wide viewing angle and improved picture quality.

[0013] Another object of the present invention is an IPS mode liquidcrystal display that can be fabricated at low cost by using a lowvoltage driving IC and by eliminating the need for a retardation film.

[0014] Additional features and advantages of the invention will be setforth in the description which follows, and in part will be apparentfrom the description, or may be learned by practice of the invention.The objectives and other advantages of the invention will be realizedand attained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

[0015] To achieve these and other advantages and in accordance with thepurpose of the present invention, as embodied and broadly described, theliquid crystal display device of the present invention includes a firstsubstrate having a first surface and a second surface, a first alignmentlayer formed on the second surface of the first substrate, a secondsubstrate having a first surface and a second surface, a secondalignment layer formed on the second surface of the second substrate, amolecular liquid crystal layer between the second surface of the firstsubstrate and the second surface of the second substrate, a pair ofelectrodes formed in parallel on the second surface of the firstsubstrate, a polarizer formed on the first surface of the firstsubstrate and having a transmittance axis, and an analyzer formed on thesecond surface of the second substrate and having a transmittance axis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this specification, illustrate embodiments of theinvention and together with the description serve to explain theprinciples of the invention. In the drawings:

[0017]FIGS. 1a, 1 b, 1 c, and 1 d schematically show cross-section andplan views of a pixel of a conventional liquid crystal display;

[0018]FIGS. 2a, 2 b, 2 c, and 2 d schematically show cross-section andplan views of a pixel of a liquid crystal display according to thepresent invention;

[0019]FIG. 3 shows the optical axes of the liquid crystal displayaccording to the present invention;

[0020]FIG. 4a shows a plan view of the liquid crystal display accordingto the present invention, and FIG. 4b shows a cross-section of thedevice taken along the line IVA-IVA of FIG. 4a;

[0021]FIG. 5 shows a pixel electrode pattern according to the presentinvention;

[0022]FIG. 6 shows a cross-sectional view taken along the line V-V ofFIG. 5;

[0023]FIG. 7 shows a driving wave pattern of a TFT LCD according to thepresent invention; and

[0024]FIG. 8 shows the relationship between the wave length of datavoltage and the transmittance of a TFT LCD according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings.

[0026] The liquid crystal display device of the present invention, asdescribed below, comprises a first transparent substrate, a secondtransparent substrate facing the first transparent substrate, analignment layer coated on the substrates, a liquid crystal layerinjected between the substrates, a polarizer attached on the outersurfaces of the substrates; a pair of electrodes formed on the firstsubstrates, and a driving circuit applying a signal voltage to theelectrodes. Liquid crystal molecules adjacent to the first substrate arerotated by applying the voltage, however, liquid crystal moleculesadjacent to the second substrate are fixed regardless of the appliedvoltage.

[0027] An embodiment of the display according to the present inventionis shown in FIGS. 2a-2 d. FIG. 2a and FIG. 2c are a cross-sectional viewand a plan view, respectively, when a driving voltage is not applied tothe display and FIGS. 2b and 2 d are a cross-sectional view and a planview, respectively, when the driving voltage is applied.

[0028] When the driving voltage is not applied, the liquid crystalmolecules adjacent to the first substrate 27 are aligned with the liquidcrystal molecules near the second substrate 26 as shown in FIGS. 2a and2 c. The liquid crystal molecules adjacent to the substrates are alignedby the alignment directions induced on alignment layers 59, 62.

[0029] The liquid crystal molecules 60 will be twisted between the twosubstrates 26, 27 by applying the driving voltage to the electrodes 48,49. The distance between two adjacent electrodes 48, 49 is less than thethickness of the liquid crystal layer 60. The retardation value iscalculated by following formula:

λ/2<Δnd≦λ.

[0030] Wherein, Δn is the dielectric anisotrophy, d is the thickness ofthe liquid crystal layer, and λ is referred as a wave length.

[0031] In addition, when the horizontal line of the liquid crystaldisplay is 0°, the angle of the electric field (θ_(FE)) applied by theelectrodes is according to the following formula:

0°<θ_(FE)<90°,

0°<θ_(FE)<−90°

[0032]FIG. 3 shows optical axes of the liquid crystal display accordingto the present invention. θ_(EL) is represented as the extensiondirection of the electrodes 48 and 59, θ_(FE) is the electric fielddirection applied by the electrodes, θ_(LC1) is the optical axisdirection of the liquid crystal molecules adjacent to the firstsubstrate and θ_(LC2) is the optical axis direction of the liquidcrystal molecules adjacent to the second substrate when the voltage isnot applied. θ_(PL1) is the principle transmission axis of a polarizer,θ_(PL2) is the principle transmission axis of an analyzer, θ_(LC1)′ isthe optical axis direction of the liquid crystal molecules adjacent tothe first substrate when the voltage is applied. The alignment directionθ_(LC1) of the first substrate is anti-parallel to the alignmentdirection θ_(LC2) of the second substrate, and the principletransmission axis θ_(PL2) of the analyzer is parallel to the alignmentdirections θ_(LC1), θ_(LC2). In addition, the principle transmissionaxis θ_(PL1) of the polarizer is perpendicular to the principletransmission axis θ_(PL2) of the analyzer.

[0033] The extension direction of the electrodes, θ_(EL), is slightlyslanted compared to the conventional extension direction which is 90°relative to the horizontal line 0° of the substrate as shown in FIG. 3.

[0034] By forming the electrodes on the slant, when the voltage is notapplied, all of the liquid crystal molecules between the two alignmentlayers 59, 62 are aligned parallel to the substrates 26, 27 and to theprinciple transmission axis θ_(PL2) of analyzer. Therefore, the viewingangle inverted areas appear at the corners of the display so that theinverted phenomenon is not remarkable. The liquid crystal is nematicwithout the need to mix a chiral dopant. The LCD shown in FIG. 2(a) is anormally black mode because the polarizer and the analyzer are crossedand the liquid crystal molecules between them are parallel to oneanother.

[0035] One of the two electrodes 48 and 49 is called a data electrodeand the other electrode is called a common electrode. The parallelelectric field 13 is formed in the θ_(FE) direction by a signal voltagebetween the data electrode, e.g., electrode 48, and the commonelectrode, e.g., the electrode 49. The parallel electric field 13 has amaximum strength adjacent to the first substrate 27, and a minimumstrength adjacent to the second substrate 26. In the middle of theliquid crystal layer 60, the parallel electric field 13 has a meanstrength defined by (E_(M)=(E₁+E₂)/2). The electric field is weaker asthe distance increases from the first substrate formed with electrodes48 and 49. Irregularities in the electric field can be avoided by makingthe thickness of the liquid crystal layer greater than the intervalbetween two electrodes.

[0036] The liquid crystal molecules 77 adjacent to the first substrate27 are rotated to the electric field direction θ_(FE) by the maximumelectric field. The rotation angle θ_(RT1) is determined by followingformula: θ_(RT1)=74 _(LC1)−θ_(LC1)′ and the maximum rotation angle isθ_(LC1)−θ_(FE).

[0037] The liquid crystal molecules 78 adjacent to the second substrate26 are subject to an electric field that is below the threshold electricfield so that the molecules retain the original direction θ_(LC2). Inthis manner, the liquid crystal molecules are continuously twistedbetween the two substrates 26, 27. Polarized light having a polarizeddirection θ_(PL1) of polarizer 63 is guided by the twisted liquidcrystal molecules 60 to the perpendicular direction parallel with theprinciple transmittance axis θ_(PL2) of the analyzer 64. As a result, awhite state is obtained by transmitting polarized light through thepolarizer 63 and the analyzer 64.

[0038] The transmittance is dependent on the twisted angleθ_(TW)=θ_(LC2)−θ_(LC1)′ that is, the transmittance is increased inproportion to the degree of the twisted angle. A grey level of theliquid crystal display is controlled by the signal voltage inducing theliquid crystal molecules to be twisted.

[0039] To improve the viewing angle, the transmittance axis of thepolarizer 63 is perpendicular to the transmittance axis of the analyzer64 (θ_(PL1)=0°, θ_(PL2)=90°), and the alignment direction θ_(LC1) of theliquid crystal molecule 77 adjacent to the first substrate 27 isanti-parallel with the alignment direction θ_(LC2) of the liquid crystalmolecule 78 adjacent to the second substrate 26 (θ_(LC1)=90°,θ_(LC2)=−90°) In addition, the extension direction θ_(EL) is angularlyoffset (for example, θ_(EL)=95°) from the electric field directionθ_(FE), which is in a direction perpendicular thereto (for exampleθ_(FE)=5°) such that extensive direction θ_(EL) is slanted. The liquidcrystal molecules 77 adjacent to the first substrate 27 are rotated from90° to 5°, while the liquid crystal molecules 78 adjacent to the secondsubstrate 26 are fixed, so that the rotation angle between the firstsubstrate 27 and the second substrate 26 is 85°.

[0040] The retardation value Δnd, which provides a maximum transmittanceto the liquid crystal layer 60, is calculated according to the followingformula: Δnd=0.74λ.The dielectric anisotrophy Δn and the thickness d ofthe liquid crystal are appropriately arranged. The dielectricanisotrophy of the liquid crystal generally used in TN mode is0.06-0.08, and the wave length of the light is 0.56 μm. When the valuesare substituted in the above formula, the thickness d should be 5.0-7.0μm.

[0041]FIG. 4a shows a plan view of a liquid crystal display according tothe present invention, and FIG. 4b shows a cross-sectional view takenalong the line IVA-IVA of FIG. 4a. The area outside of the viewing area21 is protected by a metal frame 22, an area deposited with a drivingcircuit 23 for a gate line, a driving circuit 24 for a data line, and aback light housing 25 including a back light 31. The viewing area orpresentation unit 21 is shown in FIG. 4b to successively comprise aphotoguide plate 75 including a photo-diffusion plate, a polarizer 63, afirst substrate 27, a second substrate 26, and an analyzer 64. Tocompensate for the contrast ratio of the liquid crystal display, aretardation film can be deposited either between the polarizer 63 andthe first substrate 27 or between the second substrate 26 and theanalyzer 64.

[0042] This invention can be adopted for use with a diode mode LCD usingdiodes instead of TFT, or a simple matrix LCD using a simple matrixsubstrate, as well as the TFT mode LCD which includes the firstsubstrate 27 formed with a thin film transistor and the second substrate26 formed with a color filter. In addition, this invention can beadopted to monochrome type LCD or TFT mode LCD, which includes the firstsubstrate 27 formed with a color filter and the second substrate 26formed with a thin film transistor.

[0043]FIG. 5 is a plan view showing an electrode pattern for a pixel ofthe first substrate 27. FIG. 6 is a cross-sectional view of a liquidcrystal display taken a line V-V of FIG. 5.

[0044] The liquid crystal panel is composed of two substrates 26, 27, aliquid crystal layer 60 and a spacer 65 supporting the thickness of theliquid crystal layer. The substrates 26, 27 are coated with alignmentlayers 62, 59 on their respective inner surfaces and attached withpolarizers 64, 63 on their respective outer surfaces. TFT 55 is formedbetween the first substrate 27 and alignment layer 59, and a colorfilter 61 is formed between the second substrate 26 and the alignmentlayer 62.

[0045] TFT 55 is formed in the intersection of a gate line 41 and a dataline 42, the gate line 41 extends horizontally (for example 0°) and thedata line extends vertically (for example 90°). A common line 43 passesthrough the center of the pixel in a direction parallel to the gate line41. A common electrode 49 extends from the common line 43 to theinterior inside of the pixel area in a slanted direction relative to thedata line 42. The data electrode 48 is provided parallel to commonelectrode 49 in the pixel area and is connected to a drain electrode 47of TFT 55.

[0046] A AlTa thin layer (for example Ta content about 3%) with athickness of about 0.3 μm thick is photo etched to pattern the gate line41, common line 43 and common electrode 49. Then, an AlTa oxidationlayer 52 is formed to a thickness of about 0.1 μm by anodizing thesurface of the AlTa thin layer. Both a gate insulation thick film layer57 of about a 0.3 μm of SiNx and an amorphous silicon (a-Si) layer 44are patterned by a plasma chemical vapor deposition method. A Cr thinlayer about 0.1 μm thick is deposited by a sputtering method andphotoetched to form the data line 42, a source electrode 46, and a drainelectrode 47 of the TFT 55, and the data electrode 48. The TFT 55 iscompleted by removing the N+ silicon layer within the channel of the TFT55. The intersection of the common line 43 and data electrode 48 forms astorage capacitor 53, to support an electric charge (voltage) for eachpixel. Finally, a SiNx passivation layer 58 (0.2 μm thickness) isdeposited on the entire surface.

[0047] A black matrix 51 and a color filter 61 are formed on the secondsubstrate 26. It is also possible to deposit an overcoat layer on theblack matrix 51 and the color filter 61 to provide stability andflatness for the surface. The black matrix 51 is formed with a thinlayer of width less than 10 μm, for example, 0.1 μm thick Cr/CrOx, onthe area of the gate line 41, the data line 42, and common line 43 toprevent leakage of light therefrom. The color filter 61 is repeatedlyformed with R,G,B layers in each pixel area.

[0048] In the above structure, the extension line θ_(EL) is disposed 95°relative to the horizontal line (0°), such that the electric fielddirection θ_(FE) is 5°. The extension line is extended from the 5 μmwidth data electrode 48 and the 5 μm width common electrode 49, whichare parallel to each other with a 5 μm space therebetween.

[0049] The alignment layers 59, 62 coated on the first and secondsubstrates 27, 26 are obtained by coating, for example, RN1024 (producedin NISSAN CHEMICAL CO.) to a thickness of about 0.08 μm and baking. Thealignment layer 59 coated on the first substrate 27 is rubbed in the−90° direction, and the alignment layer 62 coated on the secondsubstrate 26 is rubbed in the 90° direction. The spacer 65 can be formedfrom Micropal (produced in SEKISUI FINE CHEMICAL CO.) with an exemplary6.4 μm diameter, to maintain the liquid crystal layer 60 with a meanthickness of 6.2 μm. The liquid crystal material can be ZGS 5025(Δn=0.067; Δε=6.0; produced by CHISSO CO.). The pretilt angle of theliquid crystal is 4.8°, and the retardation value Δnd is 0.41.

[0050] The principle transmittance axis of the polarizer 63 attached onthe first substrate 27 is the horizontal direction (θ_(PL1)=0°) and thatof the analyzer 64 attached on the second substrate 26 is the verticaldirection (θ_(PL2)=90°).

[0051] The interval (horizontal spacing) between the data electrode 48and common electrode 49 is less than the thickness of liquid crystallayer 60. The retardation value of the liquid crystal layer is satisfiedwith the following formula:

[0052] λ/2<Δnd≦λ, wherein, Δn is a dielectric anisotrophy of the liquidcrystal, d is the thickness of the liquid crystal, and λ is the wavelength of the light.

[0053] The electric field direction is satisfied with the followingformula: 0°<θ_(FE)<90°.

[0054] The electro optical characteristics of the above mentioneddescribed TFT LCD are evident with reference to FIG. 7 and FIG. 8.

[0055]FIG. 7 shows the driving voltage pulse of the LCD fabricatedaccording to the present invention, wherein, the LCD has a 12.1 inchscreen, a 480×640 (R.G.B) array of pixels. The gate voltage V_(G) 71 isV_(GH)=20V, V_(GL)=0V, the width of the pulse=31 μs, and the commonvoltage V_(CO) 72 is 8V direct voltage. In addition, the data voltageV_(D) 73 is a monowave signal with a pulse width of 31μs, of which themaximum voltage is 6V, the minimum voltage is 1V, and 5V is controlledin the signal area.

[0056]FIG. 8 is a graph showing the relationship between the amplitudeof the data voltage V_(D) and the transmittance of the liquid crystalpanel. The transmittance has a maximum value of 3.6% near the 5V to datavoltage amplitude. The dotted line shows the transmittance of the LCD inwhich the interval (spacing) between two electrodes is 10 μm and thethickness of the LC layer is 6.5 μm (the interval > the thickness). Thetransmittance is increasing at 6V, and is ¼ of that illustrated by asolid line corresponding to the LCD of the present invention asdescribed herein. Therefore, the LCD according to the present inventionhas a high transmittance despite the use of a low driving voltage.

[0057] The rotation angle of the liquid crystal layer is detected by anevaluator for LCD (produced in NIHON DENSHI CO.). The results show thatthe liquid crystal molecules adjacent the second substrate 26 have atransmittance axis of 88°, and the liquid crystal molecules adjacent tothe first substrate 27 have a transmittance axis of 19°. It can beunderstood that the alignment direction of the liquid crystal moleculesnear the second substrate 26 is almost fixed, but the alignmentdirection of the liquid crystal molecules near the first substrate 27 isrotated about the anticipated angle of 16°. Therefore, the liquidcrystal molecule is twisted in the liquid crystal layer.

[0058] The driving voltage is in the range of 1.8V-5.0V, the viewingangle is over ±70° in the vertical direction and over ±70° in thehorizontal direction, wherein the contrast ratio is over 5:1, In thisrange, the gray level is not inverted. Accordingly, this LCD isremarkably improved in both vertical and horizontal viewing, can be usedwith a standard 5V driving IC, and has a front contrast ratio of 120:1.

[0059] The embodiments of the present invention have been described ashaving a 95° electric field extension line as a example. However, thedirection can be selected according to the viewing angle characteristicsthat are required.

[0060] In addition, the alignment layers for the substrates need not bethe same. For example, the alignment layer for the first substrate 27can be materials that have a low anchoring energy to allow easy rotationof the adjacent liquid crystal molecules according to the applieddriving voltage. The alignment layer for the second substrate 26 can bea polyamic-based material having a high anchoring energy liquid crystalmolecules to inhibit rotation under the influence on applied electricfield.

[0061] The present invention provides an LCD in which the data line andthe common line are deposited in a slanted orientation so that theviewing angle is improved. This provides a large rotation angle for theliquid crystal molecules to permit the thickness of the liquid crystallayer to be increased, so that a standard (low voltage) driving IC canbe used. In addition, a twisted nematic structure can be easily used inan IPS mode LCD without using a retardation film. Reliability isimproved by using conventional twisted nematic liquid crystal.

[0062] It is to be understood that the form of the preferred embodimentsof the present invention shown and described herein are to be taken as apreferred examples of the same and that various modifications andapplications may be resorted to without departing from the spirit of thepresent invention or the scope of the appended claims.

[0063] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the liquid crystal displayof the present invention without departing from the spirit or scope ofthe invention. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1-27. (Cancelled).
 28. A liquid crystal display device comprising: afirst substrate having a first surface; a second substrate having asecond surface; a liquid crystal layer formed adjacent to the firstsurface and the second surface; at least one pair of electrodes formedadjacent to said first surface; wherein a distance between the pair ofelectrodes is greater than the thickness of the liquid crystal layer.